Process for the manufacturing of dry material, by crushing, grinding or milling

ABSTRACT

The method of controlling a process for the production of dry material on a continuous feed and discharge basis such as, for example, ore pellets, in a mill unit by the crushing, grinding or milling of the dry material is achieved by measuring the amount of material being fed and the amount of process material being discharged to obtain a first signal value and a second signal value, respectively, with each of these two signal values being thereafter fed to a first control means. The degree of fill of the milling unit is electroacoustically detected to obtain a signal value therefor which is thereafter passed to a second control means. The output signal of the second control means is passed to the first control means to adjust same to a constant value. The output signal of the first control means adjusts the first control means so that the first and second signal values of the material being fed and the material being discharged will be substantially equal during the processing of the dry material.

United States Patent [191 Krijger et al.

[ PROCESS FOR THE MANUFACTURING OF DRY MATERIAL, BY CRUSHING, GRINDING 0R MILLKNG [75] Inventors: Willem Kr'riger, Uitgeest; Jacobus J.

Burger, Driehuis, both of Netherlands [73] Assignee: Koninklijke Nederlandsche l-loogoverns En Staalfabrieken N.V., Ijmuiden, Netherlands 22 Filed: Oct. 12, 1971 21 Appl. No.: 188,265

[30] Foreign Application Priority Data Oct. 12, 1970 Netherlands 7014936 [52] U.S. Cl. 241/30, 241/34 [51] Int. Cl. B02c 25/00 [58] Field of Search 241/19, 24, 30, 33, 241/34 [56] References Cited UNITED STATES PATENTS 3,314,614 4/1967 Daniel et a]. 241/34 X 2,235,928 3/1941 Hardinge 241/34 2,535,570 12/1950 Gordon 1 241/34 1,898,086 2/1933 Frisch 241/33 2,916,215 12/1959 Weston et a1 241/34 X 14 1 Jan.8, 1974 3,179,345 4/1965 Kivert et a]. 241/34 X 3,587,980 6/1971 Henne 241/30 X 3,630,457 12/1971 Forman 241/34 Primary Examiner-Granville Y. Custer, Jr. Att0rneyl-lall et al.

[5 7] ABSTRACT The method of controlling a process for the production of dry material on a continuous feed and discharge basis such as, for example, ore pellets, in a mill unit by the crushing, grinding or milling of the dry material is achieved by measuring the amount of material being fed and the amount of process material being discharged to obtain a first signal value and a second signal value, respectively, with each of these two signal values being thereafter fed to a first control means. The degree of fill of the milling unit is electroacoustically detected to obtain a signal value therefor which is thereafter passed to a second control means. The output signal of the second control means is passed to the first control means to adjust same to a constant value. The output signal of the first control means adjusts the first control means so that the first and second signal values of the material being fed and the material being discharged will be substantially equal during the processing of the dry material.

5 Claims, 2 Drawing Figures PATENTEU JAN 8 74 sum 1 OF 2 INVENTORS NM/ QM K f/J J/4c05us T, BU BY- #4, 1

ATTORNEYS PROCESS FOR THE MANUFACTURING OF DRY MATERIAL, BY CRUSHING, GRINDING OR MILLING The present invention relates to a process for the manufacture of dry material by a crushing, grinding or milling operation or the like wherein the material to be processed is fed into and out of a crushing plant on a continuous feed basis, and in which means are provided for controlling the supply of dry material to the crushing plant by controlling one of the supply or discharge flows of the dry material.

More particularly, the present invention relates to the crushing of ore for the production of ore pellets, although the invention is not to be so limited thereto.

The lump ore and sinter which has been employed heretofore as a charge for blast furnaces has been in recent years replaced by baked pellets to an ever increasing extent. It has been found that by the use of such pellets the capacity of a blast furnace may be increased and, at the same time, the quantity of required coke may be reduced. It has also been found that the best results were obtained if the pellets used had a substantial uniformity in shape, as well as having sufficient strength and porosity.

In order to produce pellets having these characteristics, it was found necessary to manufacture the pellets from very fine ore powder or ore flour which had been ground to a sufficiently fine granular size, with the grindings having a substantial uniformity in granular distribution. It has been found that such results may be best achieved if use is made of a dry crushing operation in which considerable care is exercisedto obtain a regular and uniform operation of the crushing process. It has been found that an important factor, among others, in this respect is that the feeding of the dry material to a crushing plant, such as, for example, a ball mill, is maintained at a constant value. To achieve this end, it

has been found necessary that at least one of the flows of dry material being fed to or discharged from the crushing plant be accurately adjusted in correlated relationship with another flow of dry material being fed to and discharged from said crushing plant. Earlier attempts have been made to achieve this object by measuring the flow of material being fed into and being discharged from said crushing plant to produce measure ment signals. Thereafter the measurement signals are fed to a control device, with the output signal of the control device being used for the correction of the flow of material into the processing unit. It was found that such a method was not sufficiently accurate inasmuch as the feed of dry material to the crushing plant oftentimes started to deviate from the value considered to be optimum and as a result the difference between the supply and discharge of the feed material to and from the crushing plant has a cumulative effect on the flow of feed material.

Furthermore, it was thought possible to control the feeding of the material by determining the amount of feed by measuring the sound pressure produced by the crushing plant. This was possible since, if the crushing plant is operated at a constant speed, the sound level is proportionate to the amount of the material present in the milling unit. To this end, attempts have been made to use such sound signal, measured with a microphone for instance, for the adjustment through a control device of the supply of dry material to the crushing plant. However, it was found that a control of the process by such a method was difficult to maintain in a stable state. Furthermore, an additional complication arose from the fact that the supply of dry material to the crushing plant often consisted of several flows of material, which to some degree included return flows of material which were found to be too coarse. Return flows of this type were found to be generally less uniform in size and as a result the operation of the process was difficult to control.

Considerably better results in controlling the crushing process have now been made possible by the use of the process of the present invention. The present invention includes the measuring of the supply and discharge flows of material being carried by mechanical transport systems. The measurement signals received by such measurements are adjusted in a first control device to an equal value, with the output signal of the control device being used for the correction of the supply flow which is greatest. The adjustment of the first control device is thereafter corrected by the output signal of a second control device, which adjusts the rate of feeding of the dry material to the crushing plant to a constant value on the basis of a measurement signal derived from the sound pressure produced by the crushing plant.

Therefore, the control or adjustment primarily occurs in the process of the present invention in the comparing of the flows of material which are fed to and discharged from the crushing plant, whereby the feeding is prevented from departing from a selected value by reason of the fact that the zero level of the comparing control device is adjusted on the basis ofa signal which is indicative of the rate of feed of the drymaterial. This results in an accurate and quiet adjustment of the crushing operation.

Inasmuch as the material to be crushed has a predetermined and relatively long residence time in the crushing plant, the process of the present invention will have a run-through time. Generally, in controlling processes having a run-through time by means of a continuous control means, there is the danger that considerable deviations in adjustment may occur. This is due to the factthat the amplification of the control means has to be maintained at a low value in order to prevent an excessive adjustment thereof. Excessive adjustment wherein the means to be adjusted shows great fluctuations would, in the present invention, have the disadvantages of producing a crushed product which would have a difference in granular fineness and this would detrimentally affect the quality of the pellets to be formed. I

It has been found in accordance with the present invention that the advantages encountered heretofore may be effectively eliminated if at least the first control device is of the so-called sample-type, whereby the measurement signal, or signals, of the supply flow, or flows, is, or are, retarded over a period'of time either equal to or only slightly longer than the run-through time of the material from the point of measurement to the point of discharge of the crushing plant. By having a delay which is 10 to 15% in excess of the run-through time, it has been found that slight deviations in runthrough times will not detrimentally affect the adjustment, viz., will not cause it to become out of balance. In fact, by utilizing this type of sample-type" control device, the adjustment of the next member to be adjusted for the supply flows only occurs when the effect of a preceding adjustment has become apparent. This arrangement will result in only a slight control deviation in adjustment and the process to be controlled will be greatly stabilized.

It is possible to crush dry material in an open crushing system. Any grindings which have been screened out and found to be too coarse may then be returned to the storage station from which the crushing plant is fed. However, where the crushing of ore for the production of pellets is concerned, it is particularly recommended that the crushing be carried out in a so-called closed crushing circuit, which includes a crushing plant and an air sifter. In this type of system, only the finest material screened out will be discharged from the system as crushed product, with the remainder of material being passed through the air system and being thereafter fed back to the mill for further processing. This return flow oftentimes forms an important part in the .total supply of material to be crushed to the crushing plant. A fluctuation in the crushing operation of the crushing plant also will affect the volume of the return flow of material from the air sifter, causing, in turn, the supply of material to the crushing plant to be subjected to considerable fluctuations. Accordingly, it is preferred that the effect of the fluctuating return flow is taken up in adjustment control of the crushing process. In this respect, it will be found that it is not sufficient to add the return flow as a constant signal to the flow of the material to be crushed.

ln accordance with the present invention, it will be found preferable to provide a process which is characterized by the fact that the main supply flow of material will be supplied to the crushing mill through a metering feeder. This procedure will result in a measurement signal being derived from the feeder which will be delayed over the run-through period to the crushing plant. Furthermore, a measurement signal of the return flow from the air sifter is added to the just mentioned delayed measurement signal. The sum of these signals will be further delayed for a period of time equal to the runthrough period of the material through the crushing plant, and will be subsequently fed to the first control device together with a measurement signal of the power required to effect the discharge of the crushed material from the crushing plane. The drive of the metering feeder is also controlled by the first control de vice. Since, in general, the measurement signal of the return flow of material is derived from a measurement taken just before return flow of material enters the crushing plant and almost simultaneously with the combining of the return flow and the main supply flow, it will be found unnecessary to delay the measurement signal of the return flow independently.

The main supply flow of the material to be crushed will usually not be in an absolutely dry state. Often this condition will also result from the fact that the main supply flow is drawn from a hopper which is filled, along with other things, with a slurry resulting from the subsequent manufacturing of the pellets. In this case it will be necessary to dry the contents of the crushing plant in a conventional manner, to wit, by flowing a heated drying gas in the direction of the flow of the material to be crushed through the crushing plant.

A commonly used drying process that may be employed consists of a combustion chamber being set up in front of the crushing plant in which fuel will be burned with combustion air to form hot flue gases. Care should be taken during such an operation to insure that the temperature of such flue gases will be kept within predetermined limits. These limits will be set on the one hand by the requirement that the structure of the mill or crushing plant must not be damaged by too intensive heating, while the flue gases in the plant must not reach a temperature which is below the dew point of said gases. Proposals have been made to achieve an adjustment of the temperature of the flue gas by adding cooling air to the combustion gases. In this procedure, the quantity of cooling air will be selected such that there will be, at most, a partial vacuum in the mill, i.e., a pressure not much below that of atmospheric, so that there will be little trouble encountered with the leaking in of air from the surrounding area into the crushing plant. Such pressure control can be achieved by controlling the adjustment of a chimney fan or stack fan by, for instance, a measurement of the gas pressure at the entrance of the crushing plant.

It has now been found, however, that notwithstanding the pressure control, uncontrollable fluctuations in the uniformity of the crushing operation oftentimes may occur. Investigations have shown that such fluctuations are a result of the fluctuations in the rate of speed of the drying gas passing through the crushing mill. The reason for this is that the drying gas flow also carries with it part of the crushed material along with some of the coarser fractions. Said particles carried by the gas in the discharge of the crushed material is very difficult to measure.

The uniformity of operation of the crushing process may be improved further, in accordance with the present invention, if the velocity of the gas in the crushing plant is maintained at a constant value. According to the present invention the best means to achieve this is by insuring that the gas pressures before and after passing through the crushing plant are each maintained at a constant value independently of each other by adjusting the quantity of drying gas supplied and by the adjustment of a chimney stack fan respectively. By such adjustments it will be found that the fineness of the crushed product can be maintained at a constant level. Even if the amount of material varies, the gas velocity can be maintained at a substantially constant value due to the fact that there is a constant difference in pressure being maintained between the inlet end and the outlet end of the crushing plant. It is to be noted that with the process described above, it is not only possible to control the pressure and the velocity of the drying gas, but that it is possible to control the temperature thereof. With this process, means are provided to insure that the temperature of the gas is kept constant in the proximity of the chimney stack fan.

This may be achieved by controlling the quantity of fuel supplied into the combustion chamber. It is to be noted, however, that slight fluctuations may occur in the temperature of the drying gas in the proximity of the inlet of the crushing plant. In practice it has been found that the fluctuations in temperature have no noticeable effect on the smooth and desired operation of the crushing process. I

The invention will now be more fully explained by reference to the accompanying drawings wherein:

FIG. 1 diagrammatically illustrates the flow path of the material passing through a mixing plant in which the process in accordance with the present invention may be carried out.

FIG. 2 is a diagrammatic representation of the various control circuits that may be employed in the practice illustrated in FIG. 1.

Referring now to FIG. 1, reference numeral 1 indicates a crushing plant in the form of a ball mill of a known and standard type and will therefore not be described in detail herein. Material to be crushed is drawn from a hopper 2 by means of a metering belt feeder 3. The said material is fed to the crushing plant 1 from the belt feeder 3 as a flow of material 12. The greater part of the crushed material is directly passed from the crushing plant 1 into an elevator 4 and from the elevator is passed as fiow material 5 to a second elevator 6 and is thereafter passed to an air sifter 8 as flow material 7. The finest material fractions from this flow 7 are discharged from the air sifter 8 as a flow of the final crushed product 11 which is to be subsequently processed to produce pellets. The coarsest material fractions are laterally discharged from the air sifter 8 as a flow of material 9 which, after being passed over a curved measuring guide 10, are fed back to the crushing plant 1. The measuring guide 10 is provided with a measuring system which by reason of a fluctuation in the impulse of the flow of material 9 resulting from the change of direction of movement of the material on said guide 10, can serve as a measurement for the mass flow per unit time of said flowof material 9.

The flow of material 12 from the metering feeder 3 may be changed by adjusting the drive motor 13 for the feeder 3. The elevator 4 is driven by a motor 14, the drive torque of which is measured and serves as a measurement for the flow of material 5. The degree of fill of the crushing plant 1 is determined by means of microphone 15 by which the sound emission of the jacket of the crushing plant is measured. It has been found that the electro-acoustic signal is an accurate indication for the degree of filling of the ball mill 1. In front of the crushing plant 1 a combustion chamber 16 is positioned in which the fuel is burned to produce a hot drying gas for drying the material to be crushed. The flow 18 of the fuel supplied to the combustion chamber can be adjusted with the aid of a control valve 17. The valve 17 may be adjusted by means of the adjusting motor 19. A flow of cooling air 20 is also passed into the combustion chamber 16 and is mixed therein with the burned fuel. By means of an adjustable valve 21, which can be operated by an adjusting motor 22, it will be possible to adjust the quantity of cooling air. This will also effect the quantity and the temperature of the drying gas 23 which is fed to the crushing plant 1. It is to be understood that the quantity and the temperature are also functions'of the quantity of the fuel supplied. The pressure of the drying gas in the proximity of the inlet of the crushing plant 1 is measured by means of the pressure gauge 24 which is constructed to produce an electric signal.

A similar pressure measurement is achieved in the gas flow 25 in the proximity of the outlet end of the crushing plant 1 by means of the pressure gauge 27. The gas flow 25 is fed to a coarse air sifter 28 which separates the coarsest fractions of the grindings entrained by the gas flow 25. The coarsest fractions separated by the air sifter 28 are fed back as a flow 26 and combined with the flow 5. The material freed of the coarsest fractions is thereafter conveyed to the electrofilter 29 where the dust fractions discharged as a flow of material 30 from the electro-filter 29 are combined with the material flow 11 and are discharged from the unit as final product. The temperature of the gas that leaves the electro-filter 29 as gas flow 31 is measured by instrument 32 which produces an electric control signal. The vacuum which is generated in the gas flow 31 by the chimney stack fan 33 may be controlled by means of control valve 34 which is adjusted by adjusting motor 35.

In FIG. 1 the flows of solid material are indicated by dot and dash lines, while the flows of fuel and gaseous products are indicated by dashed lines. The flow of material carried from the hopper 2 by the metering belt feeder 3 is weighed by means of the belt scale 36.

In FIG. 2 the way the electric signals produced by the various measuring devices are used to control the process is diagrammatically shown. For this purpose, at the left side in the drawing, all of the measuring devices shown in FIG. 1 which produce electrical measurement signals are presented in a vertical row. At the right in the drawing the variable speed driving motor 13 and the adjusting motors 22, 35 and 19 by which the entire process is finally controlled are shown in a vertical row. From this it will be seen that five control devices are used, viz., C 1 to C 5 inclusive, as well as two retarders 37 and 39 and an adding unit 38.

The velocity of the driving motor 13 of the metering belt feeder 3 is controlled in relationship to the signals produced by the belt scale 36, the curved measuring guide system 10, the driving motor 14, and the microphone 15. In the arrangement, the weight measurement signal of the belt scale 36 is first retarded by the retarder 37 for a period t,, which corresponds to the runthrough period of the flow of material between the belt scale and the inlet station of the crushing plant 1. The retarded signal is summed up with the signal produced by the measuring guide system 10 which is indicative of the return flow 9 from the air sifter 8. The summed up signal is retarded for a period t by the retarder 39, which period 1 corresponds to the run-through period of the material through the crushing plant 1. The combined signal which has been thus retarded indicates the mass of the delivered flow of material at the moment that the flow of material is discharged from the crushing plant 1. The mass of material is compared with the mass of the flow of material 5 which is discharged with the measured power taken up by the driving motor 14 of the elevator 4 being a measure thereof. Both signals are fed to the control device C with the output signal thereof adjusting the energy supply to the driving motor 13 of the metering belt feeder 3. The zero point of the control device C, may be adjusted by the output signal produced by control device C with the control device C reacting to the signal produced by the microphone 15. With this signal the control device C in fact controls the adjustment of control device C 1 to achieve a predetermined adjustable degree of filling of the crushing plant 1. To this end, the control device C may be adjusted to the desired degree of filling.

The gas pressures measured by the instruments 24 and 27 before and after passing through the crushing plant 1 respectively are employed to obtain an adjustment of the adjusting motors 22 and 35, with such gas pressures being used as input signals for the control devices C and C. respectively. The gas pressure measured by instrument 24 behind the combustion chamher 16 is determinative of the quantity of added cooling air into said combustion chamber. Moreover, it is also possible to maintain a constant gas pressure behind the crushing plant 1 by an adjustment of the chimney stack fan 34 by an adjustment of the control device C By selecting the adjustment of the control devices C and C the difference in pressure over the crushing plant can be adjusted to any desired constant value, with which the gas velocity through the crushing plant is also adjustable. The temperature of the drying gas after having been discharged from the electro-filter 29 is maintained at a constant value by means of the adjustment of control device C For this purpose, the fuel valve 17 is adjusted through adjusting motor 19 by the control device C It is noted that there is thus provided a crushing process which can be controlled very well and such process is not only suitable for the crushing of ore for the manufacturing of pellets, but which may be effectively used for the crushing of various other dry materials, such as coal, limestone, dolomite, and the like.

What is claimed is:

l. The method of controlling the feeding of dry material to a crushing, grinding, or milling unit on a continuous feed and discharge basis, which method comprises measuring the amount of material being fed for processing to obtain a first signal value therefor, measuring the amount of processed material discharged to obtain a second signal value therefor, passing each of said signal values to a first control means, electroacoustically detecting the degree of fill or the dry material crushing zone to obtain a signal value therefor, passing said signal value to a second control means, passing the output signal of said second control means to said first control means to adjust same to a constant value and with the output signal of said first control means thereby adjusting said first control means so that the first and second signal values of the material being fed and the material being discharged will be substantially equal during the processing,

2. The method in accordance with claim 1 wherein the first signal value measuring the amount of material fed is retarded over a time period at least equal to the run-through time period of the material from the point of measurement to the discharge point of the unit.

3. The method in accordance with claim 1 wherein the dry material is fed to the crushing unit through metering feeder means to provide the first signal value and thereafter passed on to air sifter means, passing a portion of the crushed material as a recycle feed to the crushing unit and obtaining a signal value therefor, re tarding the first signal value from the feeder means over the run-through period of the material to the crushing unit, summing up the first signal value of the feeder means with the signal value of said recycle feed to produce a further signal value, retarding the further signal value to an extent approximately the run-through period of the material through the crushing unit, feeding said retarded further signal value to said first con trol means, and controlling said metering feeder means by said first control means.

4. The method in accordance with claim 1 which includes the passing of a heated drying gas through said crushing unit in the same direction as the flow of material to be processed, the velocity of said gas being maintained at substantially a constant value.

5. The method in accordance with claim 4 wherein the pressure of the drying gas prior to passage to said crushing unit and the pressure of the drying after passing through the crushing unit and through gas draw-off means are each adjusted to obtain a constant value for each independent of one another, said adjustment being achieved by correcting the quantity of drying gas supplied and adjusting the gas draw-off means respectively.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,784,115

DATED 1 Jan. 8, 197b, mv mom Willem Kri 3 w et 21 it is cevtified thaterror appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Caption, item 73.

for "fioogoverns" read Hoogovens Signed and sealed this {0th day of June 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. WASON Commissioner of Patents and Trademarks Attesting Officer 

2. The method in accordance with claim 1 wherein the first signal value measuring the amount of material fed is retarded over a time period at least equal to the run-through time period of the material from the point of measurement to the discharge point of the unit.
 3. The method in accordance with claim 1 wherein the dry material is fed to the crushing unit through metering feeder means to provide the first signal value and thereafter passed on to air sifter means, passing a portion of the crushed material as a recycle feed to the crushing unit and obtaining a signal value therefor, retarding the first signal value from the feeder means over the run-through period of the material to the crushing unit, summing up the first signal value of the feeder means with the signal value of said recycle feed to produce a further signal value, retarding the further signal value to an extent approximately the run-through period of the material through the crushing unit, feeding said retarded further signal value to said first control means, and controlling said metering feeder means by said first control means.
 4. The method in accordance with claim 1 which includes the passing of a heated drying gas through said crushing unit in the same direction as the flow of material to be processed, the velocity of said gas being maintained at substantially a constant value.
 5. The method in accordance with claim 4 wherein the pressure of the drying gas prior to passage to said crushing unit and the pressure of the drying after passing through the crushing unit and through gas draw-off means are each adjusted to obtain a constant value for each independent of one another, said adjustment being achieved by correcting the quantity of drying gas supplied and adjusting the gas draw-off means respectively. 